The present invention relates generally to transistor switches and more particularly to three terminal, noninverting transistor switches.
Transistor switches, or transistors, are well known in the art and are commonly used in commerce. Transistors are solid-state electronic devices which are capable of amplification and switching.
A noninverting transistor switch is one type of transistor switch which is well known in the art and which is commonly used in a variety of different applications.
As an example of one potential application, a noninverting transistor switch is commonly used as a discharge switch for a power supply circuit. Specifically, the noninverting transistor switch serves to dissipate excess voltage (e.g., resulting from a power surge) which is present in the power supply circuit. As can be appreciated, failure to properly dissipate excess voltage can potentially damage a power supply circuit.
As an example of another potential application, a noninverting transistor switch is commonly used as a supervisory switch for monitoring power supplies in microprocessor and digital systems. In use, the noninverting transistor switch serves to monitor the voltage level of the power supplies and to assert a reset signal when the power supplies decline below a preset threshold.
Noninverting transistor switches typically comprise at least four terminals, one terminal being connected to an input signal, another terminal being connected to a load, another terminal being connected to ground and the last terminal being connected to a power supply in order to provide a “second” inversion for the switch.
Noninverting transistor switches which comprise only three terminals are well known and widely used in the art. Noninverting transistor switches which comprise only three terminals include a first terminal connected to an input signal, a second terminal connected to ground and a third terminal connected to a load. Noninverting transistor switches which comprise only three terminals do not require a fourth terminal connected to a power supply, thereby rendering noninverting transistor switches which comprise only three terminals more desirable than noninverting transistor switches which comprise at least four terminals.
In U.S. Pat. No. 5,134,323 to J. S. Congdon, there is disclosed a noninverting transistor switch having only three terminals, the three terminals being identified as a first terminal, a second terminal and a third terminal. The switch comprises first, second and third transistors. The first transistor includes base, emitter and collector electrodes, the base electrode being coupled to the first terminal. The second transistor includes drain, gate and source electrodes, the drain electrode being coupled to the third terminal. The third transistor includes base, emitter and collector electrodes, the emitter electrode being connected to the second terminal and the collector electrode being connected to the third terminal. The collector electrode of the first transistor is coupled to the source electrode of the second transistor and the base electrode of the third transistor. The gate electrode of the second transistor and the emitter electrode of the third transistor are coupled to the emitter electrode of the first transistor. In use, switching for the noninverting transistor switch takes place between the collector electrode and the emitter electrode of the third transistor, the collector electrode of the third transistor being connected to the third terminal of the switch and the emitter electrode of the third transistor being connected to the second terminal of the switch.
Although well known and widely used in commerce, the three terminal noninverting transistor switch which is described above and which is disclosed in U.S. Pat. No. 5,134,323 to J. S. Congdon suffers from a notable drawback. Specifically, it has been found that, upon the application of a high input signal voltage to the first terminal, the current leakage between the third terminal and the second terminal is significantly higher than zero, which is highly undesirable. In fact, it has been found that, upon the application of a high input signal voltage to the first terminal, the current leakage between the third terminal and the second terminal is approximately the IDSS value of the second transistor (the second transistor preferably being in the form of an EPI FET or channel resistor), which is highly undesirable in certain applications.